This invention relates generally to semiconductor transistors. In particular, the invention relates to heterojunction bipolar transistors. Heterojunction bipolar transistors (HBTs) offer much higher speed of operation than the more prevalent metal-oxide-semiconductor field-effect transistors (MOSFETs) or even conventional homojunction bipolar transistors, e.g., pnp or npn silicon transistors. Because HBTs offer high speed, a high current driving capability, and a low 1/f noise levels, HBTs are becoming popular for use as integrated switching devices and microwave devices in wireless communications systems and sub-systems, satellite broadcast systems, automobile collision avoidance systems, global positioning systems, and other high-frequency applications. One application in which HBT use continues to increase is in the design and manufacture of wireless electronic devices, such as wireless telephones and other like electronic devices that are capable of communicating with a network in a wireless manner. Although HBT""s offer many benefits over bipolar silicon transistors, there remains a need to improve or extend the frequency response of HBT""s.
The present invention provides an GaAs based HBT having an increased or extended frequency response. The GaAs based HBT provides an improved frequency response by reducing an emitter resistance value of the HBT.
In one embodiment of the present invention, a heterojunction bipolar transistor is provided that includes a substrate, a collector portion having at least one layer of a first material disposed on the substrate to form a first stack, a base portion having at least one layer of a second material disposed on a portion of the collector portion to form a second stack. The HBT further includes an emitter portion having at least one layer of the first material disposed over a portion of the base portion to form a third stack and a contact portion having at least one layer of the first material and at least one layer of an InxGa1-x(As1-ySby) (0 less than x less than 0.7) (0 less than y less than 0.5) material disposed over a portion of the emitter portion to form a fourth stack. The InxGa1-x(As1-ySby) (0 less than x less than 0.7) (0 less than y less than 0.5) material of the contact portion and the first material of the contact portion provide a minimal conduction band offset in the contact region of the HBT, when compared to other material types. As such, a conduction band discontinuity between in the contact region is minimized to improve the flow of electrons between the contact region and the emitter region, and, as such, realizes a reduction in the resistance value of the emitter region. The reduced resistance value of the emitter significantly increases the frequency response of the HBT. As such, the current gain cutoff frequency (fT) of the HBT is improved above 200 GHz.
In another embodiment of the present invention, a heterojunction bipolar transistor is provided that includes a substrate, a collector portion having at least one layer of a first material disposed on the substrate to form a first stack, a base portion having at least one layer of a second material disposed on a portion of the collector portion to form a second stack. The HBT further includes an emitter portion having at least one layer of a third material disposed over a portion of the base portion to form a third stack and a contact portion having at least one layer of the first material and at least one layer of an InxGa1-x(As1-ySby) (0 less than x less than 0.7) (0 less than y less than 0.5) material disposed over a portion of the emitter portion to form a fourth stack. The InxGa1-x(As1-ySby) (0 less than x less than 0.7) (0 less than y less than 0.5) of the contact portion and the first material of the contact portion provide a minimal conduction band offset in the contact region of the HBT, when compared to other material types. As such, a conduction band discontinuity in the contact region is minimized to improve the flow of electrons between the emitter region and the contact region, and as such, reduce the resistance value of the emitter region. The reduced resistance value realized by the emitter significantly increases the frequency response of the HBT. As such, the current gain cutoff frequency (fT) of the HBT is improved above 100 GHz.
In still another embodiment of the present invention a method for forming a compound semiconductor device having an extended frequency response is provided. The method includes steps for forming a collector stack having at least one layer of a first material on a substrate and forming a base stack having at least one layer of a second material on a portion of the collector stack. The method further provides the steps for forming an emitter stack having at least one layer of the first material on a portion of the base stack, and forming a contact stack having at least one layer of the first material and at least one layer of an InxGa1-x(As1-ySby) (0 less than x less than 0.7) (0 less than y less than 0.5) material on a portion of the emitter stack. The forming of the contact stack of the InxGa1-x(As1-ySby) (0 less than x less than 0.7) (0 less than y less than 0.5) material and first material allows the fabricated compound semiconductor device to realize a significant reduction in emitter resistance due to a minimal conduction band offset value in the contact stack. The resulting compound semiconductor device realizes an improved or extended fT of about 100 GHz.
In yet another embodiment of the present invention a method for forming a compound semiconductor device having an extended frequency response is provided. The method includes steps for forming a collector stack having at least one layer of a first material on a substrate and forming a base stack having at least one layer of a second material on a portion of the collector stack. The method further provides the steps for forming an emitter stack having at least one layer of a third material on a portion of the base stack, and forming a contact stack having at least one layer of the first material and at least one layer of an InxGa1-x(As1-ySby) (0 less than x less than 0.7) (0 less than y less than 0.5) material on a portion of the emitter stack. The forming of the contact stack of the InxGa1-x(As1-ySby) (0 less than x less than 0.7) (0 less than y less than 0.5) material and the first material allows the fabricated compound semiconductor device to realize a significant reduction in emitter resistance due to a minimal conduction band offset value in the contact stack. The resulting compound semiconductor device realizes an improved or extended fT greater than 100 GHz.